The invention concerns the field of machines for manufacturing containers from thermoplastic material.
More particularly, the invention concerns blow-molding machines in which containers are manufactured from plastic material, starting with a perform previously obtained, for example, by injection molding.
Such a machine has at least one blow-mold station with a blow mold in which a preform is engaged in such a way that its neck opens to the exterior of the mold. The blow-mold station has a blow pin that is movable between a retracted position and a blow position in which it is sealably held against the mold or against the neck of the preform in order to introduce the blow fluid under pressure into the preform.
Machines of this type are currently used to manufacture polyethylene terephtalate (PET) bottles and jars. In order to achieve good precision in the shape of the bottle, especially in areas of complex shapes, relatively high blow pressures are used. Thus, when the blow fluid is air, the blowing is generally done under a pressure of around 40 bars. This high pressure allows the material of the preform, which has been previously heated, to be forced against the walls that form the impression of the mold. In most machines, a stretch rod is inserted axially into the preform until it comes to rest against the closed end of the bottom of the preform at the same time as the pressurized fluid is injected. In this way the stretch rod makes it possible to better control the axial deformation of the preform during the blow-molding of the container.
In such a method of obtaining a container, the neck of the container is formed directly into its final shape during the injection molding of the preform. This neck generally is threaded to allow the cap to be screwed on to the container.
In some cases, it will be necessary to maintain a particular angular orientation of the preform with respect to the blow mold. Such is the case when neither the body of the container that one wishes to form, nor the cap with which one wishes to close it, is a body of revolution. In this case, it is generally desired that the cap, once it has been completely screwed on to the neck of the container, have a very specific orientation with respect to the body, particularly for aesthetic reasons. In this case, it is the angular arrangement of the threading of the neck with respect to the body of the container that will determine the final position of the cap when it has been screwed on.
Also, it is necessary to provide means for angularly orienting the preform with respect to the blow mold, since it is this mold that gives the body of the container its final shape.
A purpose of the invention, therefore, is to propose means of orienting the preform that are sufficiently precise to ensure a perfect angular orientation of the preform during the machine""s entire manufacturing process.
To that end, the invention proposes a machine for blow-molding thermoplastic containers, of the type having at least one blow-molding station having a blow mold in which there is a cavity in the final shape of the container to be produced, of the type in which a preform previously produced by injection molding is engaged in the cavity in such a way that only one open upper end of the preform, its neck, extends outside the mold, of the type in which the blow-molding station has a blow pin that is axially moved against the open end of the preform in order to inject pressurized air therein so as to mold the preform into the final shape of the container,
characterized in that the blow-molding station has a device for orienting the preform in the mold, said device being composed of:
a preform gripping device that can grasp the preform by its neck when it is engaged in the cavity of the mold;
a drive device for driving the gripping device in rotation to turn the preform around its axis;
a device for detecting at least one angular position of reference of the preform; and
control means that are suitable for controlling the drive device in order to cause the rotation of the gripping device until the detection device detects the angular position of reference of the preform.
According to other characteristics of the invention:
the control means are suitable for causing a rotation of the gripping device and of the preform by a predetermined offset angle from the angular position of reference;
the gripping device is carried by the blow pin;
the device for detecting the angular position of reference of the preform has an optical detector that can detect a reference mark made on the neck of the preform;
the blow pin has a body that is axially movable between an upper retracted position and a lower working position, and the body can be moved into an intermediate axial position of orientation in which the gripping device grasps the preform in order to be able to orient it angularly;
the blow pin body, when moving from its retracted position to an intermediate position, carries with it the gripping device which moves from a retracted position to a position of gripping and orienting the preform, and when the blow pin body is moved from its intermediate position to its working position, the gripping device remains stationary and is moved relative to the blow pin body by elastic return means;
at its lower end, the blow pin has a bell which, when the blow pin body is in the working position, sealably presses against the upper surface of the mold, around the neck of the preform, and the bell is axially integral with the blow pin body, but movable in rotation around the axis of the blow pin;
the bell and the gripping device are integral in rotation with each other while being axially movable with respect to each other;
the bell has an external toothed wheel by which it is driven in rotation by the drive device;
the machine has several blow-molding stations that are mounted on a rotary carrousel, and each blow-molding station has a preform orientation device.